1. Field of the Invention
This invention is in the field of motor overload protection circuits and more particularly to circuits used for protecting motors used for door opening and the like.
2. Description of the Prior Art
Motors have been used for many years to open doors of commercial establishment, and especially the larger doors such as garage doors and large commercial building doors at loading docks, warehouses, airports, truck entrances and the like. Frequently, the doors are provided with automatic opening devices which are responsive to a predetermined weight present on a pressure sensitive mat or other member in the door entryway. If the pressure sensitive mat is actuated, and the door is locked or otherwise unable to open, power is supplied to the motor but the motor is unable to operate, becomes stalled and overloaded, causing motor damage, gear drive train damage, and door stress.
Previous efforts to protect a motor against overload conditions have included thermal circuit breakers, fuses, and timer circuits which automatically turn the motor off within that time which it normally takes to open or close the door. However, these devices have deficiencies which are inherent in their operation resulting in limited use thereof. In thermal circuit breakers, once the circuit breaker reaches a circuit breaking temperature, it tends to open and close the circuit intermittently as the temperature rises and falls below the breaking temperature. In fuses, the sensitivity is limited and is susceptible to very short, transient, temporary, momentary periods of motor overload current that will cause the fuse to open before a true overload condition exists. The timing circuits that automatically turn the motor off after a predetermined time do not protect the motor during that predetermined time, during which motor damage can easily occur, and the timing period may be inadvertently restarted thus extending the time during which motor overload may continue.